This invention pertains generally to the field of cardiac pacemakers and implantable cardioverter-defibrillators having a pacemaking function. In particular, the invention relates to a system and method for testing selected operating parameters in an implanted pacemaker.
As pacemaker technology has developed, a number of standard operating modes have been developed which define how the device paces the heart. These modes are usually described by a three-letter code developed by the Inter-Society Commission for Heart Disease where each letter in the code refers to a specific function of the pacemaker. The first letter refers to which heart chambers are paced and which may be an A (for atrium), a V (for ventricle), D (for both chambers), or O (for none). The second letter refers to which chambers are sensed by the pacemaker""s sensing channels and uses the same letter designations as used for pacing. The third letter refers to the pacemaker""s response to a sensed P wave from the atrium or an R wave from the ventricle and may be an I (for inhibited), T (for triggered), D (for dual in which both triggering and inhibition are used), and O (for no response). Modem pacemakers are typically programmable so that they can operate in any mode which the physical configuration of the device will allow. Additional sensing of physiological data allows some pacemakers to change the rate at which they pace the heart in accordance with some parameter correlated to metabolic demand. Such pacemakers are called rate-adaptive and designated by a fourth letter added to the three-letter code, R.
Modem pacemakers also typically have the capability to communicate data via a radio-frequency link with an external programming device. Such data is transmitted to the pacemaker in order to program its mode of operation as well as define other operating parameters. Data transmitted from the pacemaker can be used to verify the operating parameters as well as relay information regarding the condition of both the pacemaker and the patient. Pacemaker patients are monitored at regular intervals as part of routine patient care and to check the condition of the device. Among the data which may typically be telemetered from the pacemaker are its programming parameters, an electrogram representing the electrical activity of the heart as sensed by the pacemaker, and electrical operating characteristics such as battery voltage, signal amplitude, and lead impedance.
Pacemakers have also been developed which monitor certain parameters over time while the device is functioning in the patient. Data representing these parameters can be stored in memory for later retrieval using an external programmer. Such parametric data may include the impedance of a lead that is attached to a patient""s heart and the internal impedance of a battery used to power the device. In addition, the data measurements may be taken passively so as to be synchronized with the occurrence of a cardiac event, such as using a spontaneous depolarization to measure signal amplitude from a sensing channel. Similarly, the application of a stimulation pulse by the pacemaker to the patient""s heart allows measurement of a lead impedance by measuring the current flowing through the lead when the voltage pulse of known magnitude is applied. The parametric data may be measured and stored at predetermined time intervals, as indicated by a clock within the device. Such storage of data allows the observation of trends in the data as well as detection of changes which may only occur intermittently.
Telemetry of certain types of data between the external programmer and the pacemaker can take place while the pacemaker is functioning in its currently programmed mode. Others cannot, however, because to do so might affect the operation of the pacemaker. In order to program the pacemaker, for example, it must first be put in a default pacing mode in which it can safely pace the patient while the programming parameters are adjusted. Depending upon the patient, this would typically be an asynchronous ventricular pacing mode (VOO). Other types of data which in prior art devices cannot be transmitted from the pacemaker while functioning in its programmed mode include those which derive from the pacemaker operating in a currently unprogrammed mode such as sensing signals from sensing channels not used in the current mode. Simply turning on an unused sensing channel has the potential to disturb the pacing function of the pacemaker since its programming will not allow the new incoming signal to be properly interpreted.
The present invention is directed toward a system and method which allows a pacemaker to be tested with passive measurements taken while the pacemaker is functioning in its currently programmed mode. (The term xe2x80x9cpacemakerxe2x80x9d as used herein should be taken to mean both pacemakers and any device with a pacemaking function, such as an implantable cardioverter/defibrillator with a pacemaker incorporated therein.) In contrast with prior art devices, such measurements may include signal amplitude values taken from channels which are inactive or not being used in the current operating mode. Such data may be useful to the clinician in ascertaining whether the device is capable of operating in another mode in a particular patient. In particular, the invention allows the atrial and ventricular sensing channels of a programmable pacemaker to be passively tested while the pacemaker is operating in mode where either of the sensing channels is inactive.
In accordance with the invention, a passive measurement testing mode is activated which causes a pacemaker functioning in a particular programmed mode to measure and store certain parameter values as the pacemaker transitions to test states corresponding to programmed operating states allowing such measurements to take place. In the case of parameter values not currently derivable from the current operating mode, such as the signal amplitude from a currently unprogrammed sensing channel, a virtual sensing test state corresponding to the unprogrammed channel may be interposed into the operation of the device where the pacemaker then operates as if programmed in a mode including that sensing state. The virtual sensing test state is transitioned to upon receipt of a sensing signal in the corresponding sensing channel after the expiration of any programmed refractory periods. During the virtual sensing test state an amplitude value corresponding to the sense signal is stored in memory, and no inhibition or triggering of pacing pulses is effected. The virtual sensing test state returns to the state from which it was entered.
Also in accordance with the invention, a lead impedance measuring test state may cause lead impedance values to be measured and stored when the pacemaker enters a pacing state, corresponding to the occurrence of an event which triggers output of a pacing pulse through the lead to be measured to a heart chamber according to the pacemaker""s programmed mode. An impedance value of the lead through which the pacing pulse is delivered can then be measured during output of the pacing pulse and stored in memory. In the event that no pacing pulse occurs for a given lead, either because the patient""s intrinsic rhythm during the test state is enough to inhibit pacing or because the pacemaker is not currently programmed to pace through the lead, a virtual pacing test state may be interposed into the pacemaker""s operation. During such state, which is entered upon receipt of a sense signal from a heart chamber connected to the lead to be measured, a measuring pulse is output during the chamber""s refractory period allowing the impedance of the lead to be measured and stored.